It is technically true that you do not own bitcoin and in fact noone owns bitcoins. We are -in the terminology- just allowed to "play" with them because we "own" certain private keys.
The interesting (philosophical) question is whether someone can rightfully claim exclusive ownership over certain information.
I think this is covered by law in most countries regardless. In germany this would probably fall under §202a StGB the so called "Hacker paragraph". Its basically electronically trespassing. The law just requires that you are not allowed the access the information/data by me and I secured it/them in a reasonable way.
Whether or not this would eventually end in front of a judge is something completely different.